Fibronectin-induced intracellular calcium rise in Entamoeba histolytica trophozoites: effect on adhesion and the actin cytoskeleton.

The interaction of Entamoeba histolytica trophozoites with fibronectin (FN) promotes adhesion of the protein to the cells and its later degradation by locally released proteases. Binding to FN-covered surfaces induces, in addition, the formation of actin adhesion plates and focal contacts in the amebas. The signaling mechanisms underlying the response to FN are incompletely understood. In this paper we examined the modifications of cytosolic free calcium ([Ca2+]i) induced in the trophozoites by the interaction with FN and their effect on adhesion and the actin cytoskeleton organization. FN produced a sustained rise of [Ca2+]i that could be correlated to the incremented adhesion to FN-covered surfaces. Further increments in [Ca2+]i produced by Ca2+ ionophores A23187 or ionomycin significantly increased the adhesion of trophozoites, whereas depletion of cytoplasmic Ca2+, by treatment with the ionophores in the absence of external Ca2+ or using the chelator BAPTA/AM, blocked it almost completely. To study the role of internal calcium we used the plant lactone thapsigargin, which was found to produce a transient increase of [Ca2+]i but a low stimulatory effect on adhesion and the organization of actin plates. The shifting of soluble actin to the F-actin form and the stabilization of adhesion plates and focal contacts, seen as results, of the FN stimulus, were positively influenced by rises in [Ca2+]i and negatively affected by its decrement. Additional evidence for Ca2+ -mediated signaling in the response to FN was provided by the poor adhesion and defective actin plate organization observed in trophozoites treated with calmodulin antagonists. The results presented here suggest that FN action is mainly dependent on the influx of external Ca2+.